Redox-Responsive Copper(I) Metallogel: A MetalâOrganic Hybrid

Subscriber access provided by UNIV OF WATERLOO

Article

Redox-Responsive Copper(I) Metallogel: A Metal-Organic Hybrid Sorbent for Reductive Removal of Chromium(VI) from Aqueous Solution Sougata Sarkar, Soumen Dutta, Partha Bairi, and Tarasankar Pal Langmuir, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2014 Downloaded from http://pubs.acs.org on June 19, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Redox-Responsive Copper(I) Metallogel: A Metal-Organic Hybrid Sorbent for Reductive Removal of Chromium(VI) from Aqueous Solution Sougata Sarkar‡, Soumen Dutta‡, Partha Bairi§ and Tarasankar Pal*,‡ ‡

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India

§

Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata-700032,

India E-mail: [email protected]

ABSTRACT Herein, we report a new strategy to remove toxic Cr(VI) ion from aqueous solution using metal-organic hybrid gel as sorbent. The gel could be easily synthesized from commercially available organic ligand, 2-mercaptobenzimidazole (2-MBIm) and copper(II) chloride in alcoholic medium. The synthesis involves one-electron reduction of Cu(II) to Cu(I) by 2MBIm and then gel formation is triggered through Cu(I)-ligand coordination and extensive hydrogen bonding interactions involving the “‒NH” protons (of 2-MBIm ligand), solvent molecules and chloride ions. The gel shows entangled network morphology. Different microanalytical techniques (FTIR, Powder XRD, FESEM, TEM, Rheology etc.) have been employed for complete characterizations of the gel sample. Both Cu(I) (in-situ formed) and Cl- ions trigger the gel formation as demonstrated from systematic chemical analyses. The gel also exhibits its stimuli responsive behaviour towards different interfering chemical parameters (pH, selective metal ions and anions, selective complexing agents etc.). Finally the gel shows its redox-responsive nature owing to the distinguished presence of Cu(I) metal centres throughout its structural backbone. And this indeed help in the effective removal of Cr(VI) ions from aqueous solution. Reduction of Cr(VI) to Cr(III) ions and its subsequent sorption takes place in the gel matrix. The reductive removal of Cr(VI) has been quantitatively interpreted through a set of different kinetic measurements/models and the removal capacity of the gel matrix has been observed to be ~331 mg g-1 at pH~2.7 which is admirably higher than the commonly used adsorbents. However the capacity decreases with the increase in pH of the solution. The overall removal mechanism has been clearly demonstrated. Again, the gel could also be recycled. Thus the low-cost and large-scale

ACS Paragon Plus Environment

Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

fabrication of the redox-active metallogel makes it an efficient matrix for the toxic ion removal and hence indicates the high promise of this new generation hybrid material for environmental pollution abatement.

INTRODUCTION Among the members of heavy metals causing menace to our health and environment with their acute toxicity and carcinogenicity, chromium is one of the most common elements to impose significant toxicity.1,2 A huge groundwater chromium contamination, primarily results in from its improper disposal, is reported after its widespread applications in a number of industrial processes. Among the two common oxidation states of chromium existing in aqueous solution, the environmental impacts differ significantly for Cr(VI) from Cr(III). Cr(VI) is highly toxic as well as its high ionic mobility and solution existence in a wide range of pH causes the easy contamination of ground water. Whereas, Cr(III) is relatively benign to environment, readily adsorbed in soil and also an essential trace nutrients to human health. The US Environmental Protection Agency (EPA) has also recommended the total chromium content in drinking water not to exceed a maximum limit of 0.1 mg L-1. Therefore serious attempts have been made for the removal of Cr(VI) from waste water for the past few decades through implementation of different purification methods.3-7 Among them adsorption technique has been anticipated as the cost-effective, simple and efficient one.8-19 The removal has also been achieved with reduction prompted pathway.20-23 Therefore a set of different natural and/or chemically synthesized adsorbents have been implemented for such adsorption/reduction induced Cr(VI) removal. But, the low adsorption capacity, separation and recyclability, manufacturing cost etc. of these materials sometimes limit their uses. Therefore there is still a requirement of a heterogeneous adsorbent system having high adsorption capacity. It would be more advantageous if the removal is assisted with simultaneous reduction of the adsorbed Cr(VI) ions.20-23 Here we have performed the study with a new metal-organic gel material. The gels are principally soft functional materials and may be purely organic (organogel), purely inorganic or inorganic-organic hybrid (metal ion-organic ligand gel or metallogel and metal nanoparticle-organic) materials24-28 and often bearing nanoscale morphologies. These class of materials are continuously gaining increasing attention owing to their promising applications in different branches of science and technology including catalysis, sensing, environmental pollution abatement, sorption, optoelectronics, magnetism, drug delivery and


Page 2 of 27

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

so on.29-38 In metallogel chemistry, it is anticipated that the gelators are basically the metalligand coordination complex made up with metal ions and suitably designed organic ligands and not the individual constituents (i.e. the ligand and the metal ion). Though several newly synthesized interesting ligand systems have been studied for metallogelation,26,39 comparatively little efforts have been made with the implementation of common, commercially available/natural organic molecules in making such metallogel assembly.40-46 The physicochemical stability of these metal-ligand coordination gels are principally controlled with an assortment of several essential regulating parameters for example metalligand bondings, hydrogen bondings, presence of ions, solvent composition, pH, temperature etc. and therefore the assembly of the gel matrix is frequently perturbed with making a change in either of one or more of these variables. This perturbation causes the gel stimuli responsive in nature.47 This study is receiving tremendous attention in now-a-days research with the soft materials owing to the intriguing information associated with the behaviour of the gels in a definite chemical environment. In metallogel chemistry, presence of redox-active metal centres has introduced a newer dimension and was first reported by Shinkai et al.48 in a Cu(I) containing gel of 2,2′bipyridine derived ligand. There are other very few reports on such fascinating metallogel.49 Here we have presented the spontaneous formation of a Cu(I) metal-organic gel when CuCl2 is reacted with 2-mercaptobenzimidazole (2-MBIm) in a series of suitable solvents. The formation of entangled networks is principally driven by metal-ligand coordination and hydrogen bonding interactions. The gel formation is highly selective towards Cu(II) and chloride ions. It has been examined that CuBr2 could also results gelation and no other metal chloride salts really supports this gelation. The synthesized material shows multi-responsive behaviour to different external stimuli like metal salts, selective anions, amines etc. The gel is stable at acidic/neutral pH whereas it irreversibly collapsed in alkaline pH condition. These environmental changes also cause morphological alteration of the gel matrix. The freezedried gel shows self-sustaining nature and highly porous morphology keeps the material floating over a large volume of solvent (water). Finally the presence of Cu(I) centres within the gel frameworks makes the gel redox-responsive. Therefore they could reduce Cr(VI) ions present in a solution. It has been observed that the dry gel shows very high Cr(VI) ion removal capacity (~331 mg g-1) in strongly acidic pH. The gel has got unique recyclability.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 27

Such highly efficient removal of Cr(VI) by a redox-active metal-organic gel is reported here for the first time to the best of our knowledge.

EXPERIMENTAL SECTION Materials: All the reagents were of analytical grade and used without further purification. 2mercaptobenzimidazole (2-MBIm), Cu(ClO4)2.6H2O and benzonitrile were purchased from Aldrich. All the metal salts [CuCl2.2H2O, CuBr2, Cu(NO3)2.3H2O, Cu(SO4)2.5H2O, Cu(OAc)2.H2O, KCl, MgCl2.6H2O, MnCl2.4H2O, FeCl3.6H2O, CoCl2.6H2O, NiCl2.6H2O, ZnCl2, CdCl2.H2O and HgCl2], potassium dichromate, ascorbic acid, hydrochloric acid and sodium hydroxide were purchased from Merck, India. The different solvents (methanol, ethanol,

isopropanol,

butanol,

pentanol,

octanol,

tetrahydrofuran,

acetonitrile,

dimethylformamide, dichloromethane, ethyl acetate, benzene, toluene, chloroform, diethyl ether and n-heptane) were purchased from Sisco Research Laboratories (SRL), India. All glassware were cleaned using aqua-regia, subsequently rinsed with a copious amount of double distilled water and dried well prior to use. Double distilled water was used throughout the course of the experiment. Preparation of the gel: In preparation of the metal-organic hybrid gel, 1 mL methanolic solution of 2-MBIm (6 mg; 0.04 mmol) was placed in a glass vial. To this, 1 mL methanolic solution of copper(II) chloride was added. This immediately results in a yellowish-green color solution. After little shaking, the mixture was left to stand undisturbed for 1-2 hr. Finally a white gel appeared of which the gel state was primarily confirmed (i.e. before performing the rheological experiments which ultimately supports the gel characteristics of the synthesized metalorganic hybrid materials) by the retardation of flow of the materials upon “inversion of the glass vial”. The typical gelation was found to depend on the concentration (equivalent) of CuCl2 added. Therefore ten different sets were prepared with varying concentrations from 1.0 to 0.1 molar equivalents with respect to the 2-MBIm concentration.


Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Cr(VI) removal experiments: Typically, 30 mg of the dry gel sample was dispersed in 100 mL aqueous solution of K2Cr2O7 (the source of Cr(VI) here) individually having different concentrations (from 1 mM to 0.1 mM) and then the solutions were stirred for 10 hr to reach the adsorption equilibrium. The adsorbent was then separated from the solution phase and immediately the UV-visible data of the solution was recorded and finally, the equilibrium Cr(VI) concentration for the each set was determined form standard curve. To study the pH dependence of the adsorption process, 30 mg of the dry gel sample was dispersed in 100 mL aqueous solution of 0.5 mM K2Cr2O7 individually having different pH (2.7, 3.12, 3.9, 4.71 and 7.4). The pH variation was attained meticulously with HCl or NaOH (1 M) solution. For the determination of rate equation and kinetics of the above adorption process, 30 mg of the dry gel sample was dispersed in 100 mL aqueous solution of 1 mM K2Cr2O7 and time dependent adsorption was followed spectrophotometrically (i.e. by UV-visible).

RESULTS AND DISCUSSION Typical selections of specific ligands as well as metal ions are the essential criterion in the design and engineered fabrication of metal-organic gel materials/metallogels. Among the class of such responsible ligands allowing metallogelation, urea based gelators have been playing a major role as has been well documented by Steed et al.50 and also by other research groups.51 In comparison, the chemistry of ligands encompassing thiourea-like functionality has been explored a little and thus provides a wide scope to implement such ligands as new metallogelators.52 We have recently explored46 a surface enhanced Raman scattering (SERS) based protocol for selective and sensitive detection of Cu2+ ion present in a solution. In this work, we observed that SERS spectra of a Raman reporter, 2-mercaptobenzimidazole (2-MBIm), is selectively perturbed in presence of externally added Cu2+ ions among a set of other different metal ions. Before proceeding with the above SERS experiment, we have examined the effect of the different metal ions when they are individually added in an ethanolic solution of 2-MBIm. And we surprisingly observed that only Cu2+ ion instantaneously formed a gel with the 2MBIm solution. The gelation was not observed for the other metal ions. This observation


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

suggests us that the molecule (i.e. 2-MBIm) could be a selective sensor for Cu2+ ion present in a solution. Then we have started the SERS based experiments and we succeeded in the SERS based selective and sensitive detection of Cu2+ ion. As we can observe from Figure 1 (indicated by pink colour), the molecule 2-MBIm has a thiourea-like binding site in its structural moiety. The present work reports the complete aspect of the 2-MBIm (a ligand with thiourea backbone) induced metallogel formation with CuCl2, its different physicochemical aspects and employment of the gel as a new matrix for an efficient and reductive removal of Cr(VI) ions from aqueous solution. Synthesis: The role of metal ion-ligand ratio A methanolic solution of 2-MBIm (1 mL; 0.04 mmol) when reacted with methanolic solution of copper(II) chloride (1 mL; 0.04 mmol) we observed the immediate formation yellowishwhite coloured opaque gel which on freeze drying gradually converted into a white coloured gel. Then we made a variation in the concentration (mmol equivalent) of added CuCl2. We observed that stable gel formation (as indicated by “inversion of the glass vial” method; Figure 2) takes place up to 0.6 mmol equivalent of CuCl2 (i.e. up to set V). Therefore the system presents a very low critical gelation concentration (CGC) value of 3.0 mg/mL for 2MBIm and 2.04 mg/mL for CuCl2 (considering set V). Below this concentration of CuCl2, the as-formed material either lose water or flows freely indicating the partial/weak gelation resulting from the less population of the formed gel fibers in the solvent medium. Therefore the ‘Cu(II)/2-MBIm’ stoichiometry has a critical influence on the states of the metallogelation. The key role of metal ion-ligand ratio in the process of gelation has been described also by other research groups.44 The gel obtained from (1:1) ‘Cu(II)/2-MBIm’ molar ratio in methanol was employed for other studies. Here it is worth mentioning that similar gelation could be achieved with CuBr2 in-lieu of CuCl2. Electron microscopy Figure 3 presents the typical FESEM images of the as-synthesized metallogel of set 1 (gel obtained from (1:1) ‘Cu(II)/2-MBIm’ molar ratio in methanol). The panoramic image in Figure 3a is an overview of the high-yield grown dense fibrillar network. The morphology is comprised of intertwined nanofibres which spontaneously self-assemble to form the network structure. The fibres are several micrometers long and have an approximate width of ~40-80 nm. The individual fibres are also observed to simultaneously assemble to result in bundled aggregates (Figure 3b). The fibrils are also clearly observed in the TEM image of the material


Page 6 of 27

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(Figure 3c). Successive FESEM and TEM images of the materials for other sets with varied “Cu(II)/2-MBIm” ratio are presented in Figure S2. Surprisingly, no major morphological alterations were noticed among these images. Fibrous aggregates were commonly observed to occur in every case. The energy dispersive X-ray (EDX) pattern and mapping of the metallogel confirm the presence of Cu, Cl, S and N as the key compositional elements of the hybrid material (Figure S3). Structural aspect: The selective role of ions It has been reported in literature that in solution, Cu(II) ions spontaneously reduced to Cu(I) ions with 2-MBIm.53 The associated crystal structure presents a monoclinic unit cell having tetranuclear cations [Cu4(bzimztH)10]4+, perchlorate anions and water molecules within the unit. In our case, expectedly, the N‒H protons from the imidazole unit of 2-MBIm ligand offer the favourable creation of N‒H····S and N‒H····Cl hydrogen bonds throughout the entire structure. Here π‒π stacking interaction is also expected to operating between the 2MBIm molecules as the molecules remain closely packed in the gel matrix. Finally these interactions integrate throughout and ultimately make the supramolecular gel assembly. We observed that Cu2+ salts with other counter anions like perchlorate, nitrate, sulphate and acetate could not bring this gel formation when mixed with 2-MBIm and form a clear solution or quickly results some precipitates (Figure S4a) which indicate the necessary and essential presence of chloride ion in the gelation process. It was again supported with the evidence that external addition of any ionisable chloride salt or even HCl (of definite equivalent concentration) to a clear solution mixture of Cu(ClO4)2 and 2-MBIm could successfully assist the metallogel formation. Again, as we observed that the gelation could also be achieved with CuBr2, therefore the above phenomenon of Cl- ion could be replicated for Br- ion. Therefore the above gel formation is counter anion sensitive.54-56 The sensitivity is expected to arise through hydrogen bonded gluing of the Cl- ions (in case of CuCl2) within the gel framework which is principally controlled by the size of the anion. Only Br- ion (in case of CuBr2) could be a suitable alternative. Any other common counter ion (as mentioned in the just preceding paragraph) fails to achieve the metallogelation. Bombicz et al.54 has also reported an encapsulation of chloride ions through heptacoordination mode of hydrogen bonding in one synthesized Cu(I)-thiourea complex. Our result clearly indicates that presumably other ions fail to meet the optimum size condition and thus, are not suitably hydrogen bonded to achieve the metallogel formation. Steed et al. have


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

described the selective role of anions in metallogelation for different urea functionalized gelators.50 In a likewise fashion to the counter anion dependency, other common metal chloride salts were also incapable to introduce any gelation with 2-MBIm (except only CuCl2) under the same reaction environment (Figure S4b). Similar conclusion could be drawn for CuBr2. Therefore simultaneous presence of both Cu(II) and Cl- ions (in case of CuCl2‒2-MBIm system) or Cu(II) and Br- ions (in case of CuBr2‒2-MBIm system) were essential to initiate the metallogel formation.56 Solvent effect The solvent gelation ability of the bicomponent gelator (2-MBIm and CuCl2) system was studied in a couple of pure and mixed solvents. Fibrous morphology was observed in all these cases (Figure S5). However the solvents employed were either polar protic or polar aprotic in nature due to the insolubility of the gelator(s) in common nonpolar solvents. It is now well known that large volumes of appropriate solvent molecules are trapped in the interstitial sites of the assembled nanofibres and directs the gel formation. Table S1 presents the complete observation of the experimental results which clearly depicts that delicate balance of the Kamlet-Taft solvent parameters45 primarily contribute to the state of the gelation. We also observed that solid state grinding of CuCl2 and 2-MBIm resulted in a greenish powder which remains stable for days together in well-stoppered condition but upon addition of methanol, this immediately turns into a white gelatinous product (Figure S6). These observations thus clearly indicate that suitably chosen solvent molecules play the another essential role in assistance with the Cu2+ and Cl- ions in the formation of the ‘Cu(I)’ gel networks. Rheology Supramolecular Gels are viscoelastic materials and can store/squander energy which are characterized by storage (G') and loss modulus (G''), respectively. In the gel state G'(ω) > G'' (ω) and G'(ω) ~ ω0, where ω is the angular frequency. Apart from the verification of gel formation, the mechanical strength, fragility (σ*, minimum stress required to rupture the gel), elasticity (G' - G'') and stiffness (G'/G''), of the gel could be measured from the rheological data. The dynamic frequency sweep experiment of the as-synthesized gel is shown in Figure 4a and a wide linear viscoelastic region (LVR) with frequency is observed. A considerably higher G' value (20860 Pa) than that of G'' (6261 Pa) also observed which confirm its gel nature. The oscillator stress sweep experimental data is shown in Figure 4b and we observed


Page 8 of 27

Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

a very high fragility (σ*) 500 Pa which indicating very high stress is required to rupture the gel. The mechanical strength (G'), elasticity (G'-G'') and stiffness (G'/G''), of the supramolecular gel are 20860 Pa, 14599 Pa and 3.3 respectively. Self-sustaining nature The as-prepared metal-organic gel shows self-sustaining nature as it is possible to synthesize the gel in large-scale as well as different shapes could be executed with the bulk gel (Figure S7a-b). The freeze-dried bulk gel also forms monoliths (Figure S7c-d) and it was found that the freeze-dried gel could easily hold the weight of a water-filled beaker (100 mL) for several hours and withstand compression (Figure S7e-g).57 The freeze-dried gel also shows highly porous nature as confirmed when a monolith of weight ~32 g shows its spontaneous floating behaviour over water (Figure S7h-i). This probably owing to its entrapment of large volume of solvent (methanol; density 0.7918 g cm-3) molecules which effectively reduces the density of the bulk gel and helps in its floating activity (Please see the submitted video file of the floating monolith). Here it is also important to note that though the gel contains Cu(I) ions but still remains stable to oxidation for couple of days in aqueous environment. Redox-responsive behaviour The Cu(I) centres within the framework of the gel matrix made our as synthesized gel highly redox-active. Therefore when a piece of the bulk gel (freeze-dried) was put into an aqueous solution of potassium dichromate, the yellow colour of the solution gradually disappears and the white colour of gel matrix slowly acquires green colour and finally turned into deep green (Figure 5). It is anticipated that Cr(VI) ions (more precisely Cr2O72- ions) first adsorbed in the porous gel network and gets reduced to Cr(III) ions through electron transfer from the Cu(I) centres and the Cu(I) ions are subsequently oxidized to Cu(II) ions. The Cr(III) ions thereafter remains bound in the gel matrix through associated coordination from the imidazolium nitrogen donors and results in greenish coloration of the gel matrix.18 The above oxidation of Cu(I) to Cu(II) ions renders the gel inactive for further use in Cr(VI) removal. Here it is worth mentioning that post treatment of this oxidised gel matrix with a solution of ascorbic acid (AA) could instantaneously reduce back the Cu(II) ions to Cu(I) ions and thus the gel matrix could be rejuvenated again. In this way the gel could be recycled. This whole redox-switchable phenomenon happens at room temperature and can be performed repeatedly.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Reductive removal of Cr(VI) ions from aqueous solution: Kinetic and mechanistic aspects Finally this behaviour of the Cu(I) metallogel prompted us to implement this material in reductive removal of Cr(VI) ions from aqueous solution. A set of kinetic experiments were thus performed to evaluate the adsorption kinetics, adsorption isotherm, effect of pH on adsorption capacity and recyclability of the metal-organic adsorbent etc. Figure 6a presents the kinetics of the removal of Cr(VI) ion from aqueous solution. We observe that the rate of adsorption is rapid for first few hours and then the rate gradually becomes steady with time as well as the removal time is consecutively less for higher amount of the material. Finally the adsorption kinetics could be well addressed with pseudo-secondorder kinetic model58 [Figure 6b; which is also corroborated from the correlation coefficient value (R2 = 0.994)]. The corresponding equation is expressed by: t/Qt = 1/k2(Qe)2 + t/Qe where Qe and Qt are the amount of Cr(VI) adsorbed on unit mass of the adsorbent when the concentration of Cr(VI) in solution is in equilibrium and at time t (min) respectively and k2 (g mg-1 min-1) is the rate constant of the kinetic model. From Figure 6b, the rate constant has been determined to be 5.15x10-5 g mg-1 min-1. The experimental adsorption capacity (qe) is also observed to be very close to the theoretical adsorption capacity (calculated from the slope of Figure 6b). This again substantiates the above adsorption kinetics is fitted well with the pseudo-second-order model. The Cr(VI) removal efficiency of the gel material can be illustrated with adsorption isotherm and therefore the Langmuir and Freundlich isotherm models have been implemented to simulate the adsorption.58 The Langmuir isotherm equation is expressed by: Qe = bQmCe/(1+bCe) where Qe (mg g-1) presents the amount of the Cr(VI) adsorbed per unit weight of the adsorbent (here the dried gel material) at equilibrium, Ce (mg L-1) is the equilibrium concentration of the Cr(VI), Qm (mg g-1) presents the maximum adsorption capacity of the adsorbent and b is the Langmuir adsorption constant. The Freundlich isotherm equation is expressed by: Qe = kCe1/n


Page 10 of 27

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

where k is the Freundlich constant, 1/n is the adsorption intensity; Qe (mg g-1) and Ce have the similar meaning as above. From the Langmuir isotherm equation, we obtain Ce/Qe = {1/(bQm) + (Ce/Qm)} which indicates that plot of Ce/Qe vs. Ce will be linear if the adsorption follows Langmuir isotherm equation whereas, from Freundlich isotherm, we have lnQe = lnk + (1/n) lnCe which indicates that plot of lnQe vs. lnCe would be linear if the adsorption kinetics follows Freundlich isotherm. Figure 7a presents the plot of Ce/Qe vs. Ce whereas Figure 7b presents the plot of lnQe vs. lnCe for the adsorption of Cr(VI) at pH~3.12. From these plots we observe that the adsorption isotherm fitted well to the Langmuir model with a correlation coefficient (R2) value of 0.996 compared to that of Freundlich model with R2 = 0.432. This observation suggests that the Cr(VI) adsorption in the gel matrix is a monolayer adsorption phenomena which may be attributed to the homogenous distribution of the Cu(I) metal-organic coordination unit throughout the gel fibres. Figure 8 presents the adsorption isotherm plot of Qe vs. Ce at pH ~3.1 which merely follows the Langmuir model and based on this isotherm we have the maximum adsorption capacity (Qm) to be ~202 mg g-1. Here it is worth mentioning that this value is quite higher than previously reported values on the removal of hexavalent chromium ion from aqueous solution by diverse materials.7 The reductive removal of Cr(VI) by the gel material was observed to be critically influenced by the pH of the solution medium. Low/acidic pH favours the more efficient removal over higher pH of the medium. Figure 9 presents the variation in Qe (in mg g-1) with pH of the medium which clearly suggests that the maximum adsorption capacity reaches at pH~2.7 (~331 mg g-1) which gradually falls down to 81.3 mg g-1 at pH~7.5. Thus the highest adsorption capacity of the material was obtained at pH ~2.7. This feature has also been highlighted in previous reports.14 The pH dependent presence of different Cr(VI) species in aqueous solution contributes to this effect. It is well known that the dichromate ion in alkaline solution changes to the tetrahedral chromate ion and on lowering the pH, the yellow chromate solution goes over to the orange dichromate. CrO42- + H+ = CrO3(OH)-;

2CrO3(OH)- = H2O + Cr2O72-


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Therefore, CrO42- (in alkaline pH), Cr2O72- (in neutral pH) and HCrO4- (in acidic pH) are the major/predominant ions present in aqueous solution depending upon the pH of the solution. Our experimental result indicates that adsorption of HCrO4- is mostly favoured by the gel material. This may be explained considering protonation effect of the gel networks. The gel is enriched with sufficient ‒NH groups of the imidazole unit and therefore in acidic pH environment, these groups are favourably protonated. These enriched presences of “‒NH2+” functionalities throughout the gel matrix favour the adsorption of the anionic HCrO4- through ionic interactions. In higher pH, due to the lack in protonation, the positive charge in the matrix is substantially reduced which in-turn results in poor adsorption of the corresponding ionic species. Therefore the relative higher quantity reductive removal of Cr(VI) by the Cu(I) gel material in acidic pH can be understood. We have performed XPS analysis of the gel sample (thoroughly washed and dried after Cr(VI) removal) where we observed the presence peaks at 578.9 eV (Cr2p3/2) and 587.8 (Cr2p1/2) eV respectively which are due to the Cr(III) ion. Thus the simultaneous reduction of the adsorbed Cr(VI) to Cr(III) is once again substantiated. After first cycle of Cr(VI) removal, the Cu(I) ions of the gel matrix get oxidized by Cr(VI) and this oxidation renders the gel inactive for further use in Cr(VI) removal. Here it is worth mentioning that post treatment of this oxidised gel matrix with a solution of ascorbic acid (AA) could instantaneously reduce back the Cu(II) ions to Cu(I) ions and thus the gel matrix could be rejuvenated again. The material after its first cycle was treated with ascorbic acid solution, thoroughly washed with water, followed by a little acetone. This regenerated gel was reused again for another new set of dichromate removal. For each case we have finally calculated Qe (mg g-1) where we found that after 1st cycle it was 202 mg g-1 and after 2nd cycle the value of Qe reaches 190 mg g-1. Therefore even after 2nd cycle 94% removal of Cr(VI) was achieved with the reused gel material. In this way the gel could be recycled. Therefore the easily-fabricated nanoscale metal-organic material could be a potential candidate for simultaneous adsorption and reductive removal of Cr(VI) from aqueous solution. Here it is worth mentioning that such highly efficient removal of Cr(VI) by a redoxactive metal-organic gel is reported here for the first time to the best of our knowledge.


Page 12 of 27

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

CONCLUSIONS In conclusion, we have reported an efficient reductive removal recipe of toxic Cr(VI) ions from aqueous solution employing a redox active metal-organic gel. The gel could be easily synthesized

using

CuCl2

and

a

commercially

available

organic

ligand,

2-

mercaptobenzimidazole (2-MBIm) in alcoholic solvent where Cu(II) ions are reduced to Cu(I) and in-turn make the gel redox responsive. The gelation study has been carried out with different ‘metal ion/ligand’ concentration ratio and also in a couple of different solvent systems. The gel shows the presence of fibrous-network morphology throughout, which is formed with the participation of metal-ligand i.e. Cu(I)-ligand coordination, extensive hydrogen bonding interactions involving the “‒NH” protons (of 2-MBIm ligand), solvent molecules and chloride ions, π‒π interactions etc. A set of different chemical stimuli were observed to affect the stability of the gel phase. Finally thorough presence of Cu(I) centres in the gel matrix successfully reduce Cr(VI) ions to Cr(III) when the gel remains incubated in a solution of Cr2O72-. The Cr(VI) removal capacity of the dried gel matrix was observed to be ~331 mg g-1 at pH~2.7 which has been determined through a set of different kinetic measurements/models. The removal mechanism has been demonstrated. Again, the gel could also be reused. Therefore the easily synthesized redox-active metallogel turns to be an efficient matrix for the reductive removal of environmentally toxic and hazardous Cr(VI) ions. The study thus presents a newer approach towards removal of Cr(VI) ions with a metalorganic hybrid material and hence would be promising for the treatment of Cr(VI)-containing wastewater. SUPPORTING INFORMATION Analytical methods; FTIR analyses of the metallogel (Figure S1) and discussion; FESEM images of the metallogel having different ‘Cu(II)/2-MBIm’ ratio (Figure S2); EDX pattern and elemental mapping analysis of metallogel (Figure S3); ion sensitivity of the gelation (Figure S4); FESEM images of the metal-organic gels prepared in a couple of pure and mixed solvents (Figure S5); solid-state synthesis (Figure S6); self-sustaining nature (Figure S7); effect of interfering ions (Figure S8) and discussion; stimuli responsive behaviour (Figure S9) and discussion; pH effect (Figure S10) and discussion; low and high angle powder X-ray diffraction pattern (Figure S11); DRS spectrum (Figure S12); Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACKNOWLEDGEMENTS The authors are thankful to the CSIR, UGC, DST and Indian Institute of Technology, Kharagpur for financial assistance. REFERENCES (1) Baruthio, F. Toxic Effects of Chromium and its Compounds. Biol. Trace Elem. Res. 1992, 32, 145-153. (2) Wise, S. S.; Shaffiey, F.; LaCerte, C.; Goertz, C. E. C. ; Dunn, J. L.; Gulland, F. M. D.; Aboueissa, A. ‒M.; Zheng, T.; Sr. Wise, J. P. Particulate and Soluble Hexavalent Chromium are Cytotoxic and Genotoxic to Steller Sea Lion Lung Cells. Aquat. Toxicol. 2009, 91, 329-335. (3) Barrera-Díaz, C. E.; Lugo-Lugo, V.; Bilyeu, B. A Review of Chemical, Electrochemical and Biological Methods for Aqueous Cr(VI) Reduction. J. Hazard. Mater. 2012, 223-224, 1-12. (4) Gheju, M.; Balcu, I. Removal of Chromium from Cr(VI) Polluted Wastewaters by Reduction with Scrap Iron and Subsequent Precipitation of Resulted Cations. J. Hazard. Mater. 2011, 196, 131-138. (5) Guertin, J.; Jacobs, J. A.; Avakian, C. P. Chromium(VI) Handbook; CRC Press, 2004. (6) Djouider, F. Radiolytic Formation of Non-toxic Cr(III) from Toxic Cr(VI) in Formate Containing Aqueous Solutions: A System for Water Treatment. J. Hazard. Mater. 2012, 223-224, 104-109. (7) Owlad, M.; Aroua, M. K.; Daud, W. A. W.; Baroutian, S. Removal of Hexavalent Chromium-Contaminated Water and Wastewater: A Review. Water, Air, Soil Pollut. 2009, 200, 59-77.


Page 14 of 27

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(8) Rosales-Landeros, C.; Barrera-Díaz, C.; Bilyeu, B.; Guerrero, V.; Núñez, F. A Review on Cr(VI) Adsorption Using Inorganic Materials. American Journal of Analytical Chemistry 2013, 4, 8-16. (9) Panda, M.; Bhowal, A.; Datta, S. Removal of Hexavalent Chromium by Biosorption Process in Rotating Packed Bed. Environ. Sci. Technol. 2011, 45, 8460-8466. (10)

Wang, Y.; Zou, B.; Gao, T.; Wu, X.; Lou, S.; Zhou, S. Synthesis of Orange-

like Fe3O4/PPy Composite Microspheres and their Excellent Cr(VI) Ion Removal Properties. J. Mater. Chem. 2012, 22, 9034-9040. (11)

Chen, L. –F.; Liang, H. –W.; Lu, Y.; Cui, C. –H.; Yu, S. –H. Synthesis of an

Attapulgite

Clay@Carbon

Nanocomposite

Adsorbent

by

a

Hydrothermal

Carbonization Process and Their Application in the Removal of Toxic Metal Ions from Water. Langmuir 2011, 27, 8998-9004. (12)

Baikousi, M.; Bourlinos, A. B.; Douvalis, A.; Bakas, T.; Anagnostopoulos, D.

F.; Tuček, J.; Safářová, K.; Zboril, R.; Karakassides, M. A.

Synthesis and

Characterization of γ-Fe2O3/Carbon Hybrids and Their Application in Removal of Hexavalent Chromium Ions from Aqueous Solutions. Langmuir 2012, 28, 3918-3930. (13)

Zhang, L. –H.; Sun, Q.; Liu, D. –H.; Lu, A. –H. Magnetic Hollow Carbon

Nanospheres for Removal of Chromium Ions. J. Mater. Chem. A 2013, 1, 9477-9483. (14)

Fan, L.; Luo, C.; Sun, M.; Qiu, H. Synthesis of Graphene Oxide Decorated

with Magnetic Cyclodextrin for Fast Chromium Removal. J. Mater. Chem. 2012, 22, 24577-24583. (15)

Wei, Z.; Xing, R.; Zhang, X.; Liu, S.; Yu, H.; Li, P. Facile Template-Free

Fabrication of Hollow Nestlike α-Fe2O3 Nanostructures for Water Treatment. ACS Appl. Mater. Interfaces 2013, 5, 598-604.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(16)

Page 16 of 27

Zhu, J.; Wei, S.; Gu, H.; Rapole, S. B.; Wang, Q.; Luo, Z.; Haldolaarachchige,

N.; Young, D. P.; Guo, Z. One-Pot Synthesis of Magnetic Graphene Nanocomposites Decorated with Core@Double-shell Nanoparticles for Fast Chromium Removal. Environ. Sci. Technol. 2012, 46, 977-985. (17)

Bhowmik, K.; Mukherjee, A.; Mishra, M. K.; De, G. Stable Ni Nanoparticle-

Reduced Graphene Oxide Composites for the Reduction of Highly Toxic Aqueous Cr(VI) at Room Temperature. Langmuir 2014, 30, 3209-3216. (18)

Shen, H.; Chen, J.; Dai, H.; Wang, L.; Hu, M.; Xia, Q. New Insights into the

Sorption

and

Detoxification

of

Chromium(VI)

by

Tetraethylenepentamine

Functionalized Nanosized Magnetic Polymer Adsorbents: Mechanism and pH Effect. Ind. Eng. Chem. Res. 2013, 52, 12723-12732. (19)

Miretzky, P.; Cirelli, A. F. Cr(VI) and Cr(III) Removal from Aqueous

Solution by Raw and Modified Lignocellulosic Materials: A Review. J. Hazard. Mater. 2010, 180, 1-19. (20)

Shi, L.; Zhang, X.; Chen, Z. Removal of Chromium(VI) from Wastewater

using Bentonite-Supported Nanoscale Zero-Valent Iron. Water Res. 2011, 45, 886892. (21)

Ai, Z.; Cheng, Y.; Zhang, L.; Qiu, J. Efficient Removal of Cr(VI) from

Aqueous Solution with Fe@Fe2O3 Core-Shell Nanowires. Environ. Sci. Technol. 2008, 42, 6955-6960. (22)

Lai, K. C. K.; Lo, I. M. C. Removal of Chromium (VI) by Acid-Washed Zero-

Valent Iron under Various Groundwater Geochemistry Conditions. Environ. Sci. Technol. 2008, 42, 1238-1244.


Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(23)

Ke, Z.; Huang, Q.; Zhang, H.; Yu, Z. Reduction and Removal of Aqueous

Cr(VI) by Glow Discharge Plasma at the Gas-Solution Interface. Environ. Sci. Technol. 2011, 45, 7841-7847. (24)

Abdallah, D. J.; Weiss, R. G. Organogels and Low Molecular Mass Organic

Gelators. Adv. Mater. 2000, 12, 1237-1247. (25)

Buerkle, L. E.; Rowan, S. J. Supramolecular Gels formed from Multi-

component Low Molecular Weight Species. Chem. Soc. Rev. 2012, 41, 6089-6102. (26)

Zhang, J.; Su, C.-Y. Metal-organic Gels: From Discrete Metallogelators to

Coordination Polymers. Coord. Chem. Rev. 2013, 257, 1373-1408 and references there-in. (27)

Basit, H.; Pal, A.; Sen, S.; Bhattacharya, S. Two-Component Hydrogels

Comprising Fatty Acids and Amines: Structure, Properties, and Application as a Template for the Synthesis of Metal Nanoparticles. Chem. Eur. J. 2008, 14, 65346545. (28)

Mondal, C.; Ganguly, M.; Pal, J.; Sahoo, R.; Sinha, A. K.; Pal, T. Pure

Inorganic Gel: A New Host with Tremendous Sorption Capability. Chem. Commun. 2013, 49, 9428-9430. (29)

Tam, A. Y. –Y.; Yam, V. W. –W. Recent Advances in Metallogels. Chem.

Soc. Rev. 2013, 42, 1540-1567 and references there-in. (30)

Sangeetha, N. M.; Maitra, U. Supramolecular Gels: Functions and Uses.

Chem. Soc. Rev. 2005, 34, 821-836 and references there-in. (31)

Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Organogels as Scaffolds for

Excitation Energy Transfer and Light Harvesting. Chem. Soc. Rev. 2008, 37, 109-122 and references there-in.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(32)

Díaz, D. D.; Kühbeck, D.; Koopmans, R. J. Stimuli-responsive Gels as

Reaction Vessels and Reusable Catalysts. Chem. Soc. Rev. 2011, 40, 427-448 and references there-in. (33)

Lee, H.; Jung, S. H.; Han, W. S.; Moon, J. H.; Kang, S.; Lee, J. Y.; Jung, J. H.;

Shinkai, S. A Chromo-Fluorogenic Tetrazole-Based CoBr2 Coordination Polymer Gel as a Highly Sensitive and Selective Chemosensor for Volatile Gases Containing Chloride. Chem. Eur. J. 2011, 17, 2823-2827. (34)

Zhou, C.; Gao, W.; Yang, K.; Xu, L.; Ding, J.; Chen, J.; Liu, M.; Huang, X.;

Wang, S.; Wu, H. A Novel Glucose/pH Responsive Low-Molecular-Weight Organogel of Easy Recycling. Langmuir 2013, 29, 13568-13575. (35)

Paul, M.; Adarsh, N. N.; Dastidar, P. CuII Coordination Polymers Capable of

Gelation and Selective SO4–2 Separation. Cryst. Growth Des. 2012, 12, 4135-4143. (36)

Adhikari, B.; Palui, G.; Banerjee, A. Self-assembling Tripeptide based

Hydrogels and Their Use in Removal of Dyes from Waste-water. Soft Matter 2009, 5, 3452-3460. (37)

Roubeau, O.; Colin, A.; Schmitt, V.; Clérac, R. Thermoreversible Gels as

Magneto-Optical Switches. Angew. Chem. Int. Ed. 2004, 43, 3283-3286. (38)

Naskar, J.; Palui, G.; Banerjee, A. Tetrapeptide-Based Hydrogels: for

Encapsulation and Slow Release of an Anticancer Drug at Physiological pH. J. Phys. Chem. B 2009, 113, 11787-11792. (39)

Byrne, P.; Lloyd, G. O.; Applegarth, L.; Anderson, K. M.; Clarke, N.; Steed, J.

W. Metal-induced Gelation in Dipyridyl Ureas. New J. Chem. 2010, 34, 2261-2274. (40)

Imaz, I.; Rubio-Martínez, M.; Saletra, W. J.; Amabilino, D. B.; Maspoch, D.

Amino Acid Based Metal-Organic Nanofibers. J. Am. Chem. Soc. 2009, 131, 1822218223.


Page 18 of 27

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(41)

Odriozola, I.; Loinaz, I.; Pomposo, J. A.; Grande, H. J. Gold-glutathione

Supramolecular Hydrogels. J. Mater. Chem. 2007, 17, 4843-4845. (42)

Casuso, P.; Carrasco, P.; Loinaz, I.; Cabañero, G.; Grande, H. J.; Odriozola, I.

Argentophilic Hydrogels: Elucidating the Structure of Neutral versus Acidic Systems. Soft Matter 2011, 7, 3627-3633. (43)

Banerjee, S.; Kandanelli, R.; Bhowmik, S.; Maitra, U. Self-organization of

Multiple Components in a Steroidal Hydrogel Matrix: Design, Construction and Studies on Novel Tunable Luminescent Gels and Xerogels. Soft Matter 2011, 7, 82078215. (44)

Bairi, P.; Roy, B.; Nandi, A. K. pH and Anion Sensitive Silver(I) Coordinated

Melamine Hydrogel with Dye Absorbing Properties: Metastability at Low Melamine Concentration. J. Mater. Chem. 2011, 21, 11747-11749. (45)

Saha, S.; Schön, E. –M.; Cativiela, C.; Díaz, D. D.; Banerjee, R. Proton-

Conducting Supramolecular Metallogels from the Lowest Molecular Weight Assembler Ligand: A Quote for Simplicity. Chem. Eur. J. 2013, 19, 9562-9568. (46)

Sarkar, S.; Pradhan, M.; Sinha, A. K.; Basu, M.; Pal, T. Selective and

Sensitive Recognition of Cu2+ in an Aqueous Medium: A Surface-Enhanced Raman Scattering (SERS)-Based Analysis with a Low-Cost Raman Reporter. Chem. Eur. J. 2012, 18, 6335-6342. (47)

Segarra-Maset, M. D.; Nebot, V. J.; Miravet, J. F.; Escuder, B. Control of

Molecular Gelation by Chemical Stimuli. Chem. Soc. Rev. 2013, 42, 7086-7098. (48)

Kawano, S. –i.; Fujita, N.; Shinkai, S. A Coordination Gelator That Shows a

Reversible Chromatic Change and Sol-Gel Phase-Transition Behavior upon Oxidative/Reductive Stimuli. J. Am. Chem. Soc. 2004, 126, 8592-8593.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(49)

Miao, W.; Zhang, L.; Wang, X.; Cao, H.; Jin, Q.; Liu, M. A Dual-Functional

Metallogel of Amphiphilic Copper(II) Quinolinol: Redox Responsiveness and Enantioselectivity. Chem. Eur. J. 2013, 19, 3029-3036. (50)

Steed, J. W. Anion-tuned Supramolecular Gels: A Natural Evolution

from Urea Supramolecular Chemistry. Chem. Soc. Rev. 2010, 39, 3686-3699. (51)

Esch, J. H. v.; Schoonbeek, F.; Loos, M. d.; Kooijman, H.; Spek, A. L.;

Kellogg, R. M.; Feringa, B. L. Cyclic Bis-Urea Compounds as Gelators for Organic Solvents. Chem. Eur. J. 1999, 5, 937-950. (52)

George, M.; Tan, G.; John, V. T.; Weiss, R. G. Urea and Thiourea

Derivatives as Low Molecular-Mass Organogelators. Chem. Eur. J. 2005, 11, 32433254. (53)

Raper, E. S.; Creighton, J. R.; Wilson, J. D.; Clegg, W.; Milne, A.

Preparation, Spectroscopy and Crystal Structure of a Novel Tetranuclear Copper(I) Benzimidazoline-2-thione

Complex:

cyclo-Hexakis-µ2-(benzimidazoline-2-thione)

Tetrakis [Benzimidazoline-2-thione coper(I)] Perchlorate Quatrodeca Hydrate. Inorg. Chim. Acta 1988, 149, 265-271. (54)

Bombicz, P.; Mutikainen, I.; Krunks, M.; Leskelä, T.; Madarász, J.; Niinistö,

L. Synthesis, Vibrational Spectra and X-ray Structures of Copper(I) Thiourea Complexes. Inorg. Chim. Acta 2004, 357, 513-525. (55)

Piepenbrock, M. –O. M.; Lloyd, Clarke, N.; Steed, J. W. Metal Ion and Anion-

based “Tuning” of a Supramolecular Metallogel. Langmuir 2009, 25, 8451-8456. (56)

Džolić, Z.; Cametti, M.; Milić, D.; Žinić, M. The Formation of CuCl2-

Specific Metallogels of Pyridyloxalamide Derivatives in Alcohols. Chem. Eur. J. 2013, 19, 5411-5416.


Page 20 of 27

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(57)

Saha, S.; Bachl, J.; Kundu, T.; Díaz, D. D.; Banerjee, R. Amino Acid-based

Multiresponsive Low-molecular Weight Metallohydrogels with Load-bearing and Rapid self-healing Abilities. Chem. Commun. 2014, 50, 3004-3006. (58)

Liu, Y.; Wang, Y.; Zhou, S.; Lou, S.; Yuan, L.; Gao, T.; Wu, X. Shi, X.;

Wang, K. Synthesis of High Saturation Magnetization Superparamagnetic Fe3O4 Hollow Microspheres for Swift Chromium Removal. ACS Appl. Mater. Interfaces 2012, 4, 4913-4920.


Langmuir

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

Figure Captions H N

N

H2N S

SH

S

N H

N H Thiol form of 2-MBIm

H2N Thiourea

Thione form of 2-MBIm

Figure 1. Thiol and thione tautomeric forms of 2-mercaptobenzimidazole (2-MBIm). The ‘thione’ form clearly illustrates the presence of ‘thiourea-like’ binding unit in its structural backbone (as indicated by pink colour).

I

II

III

IV

V

VI

VII

VIII

IX

X

Figure 2. CuCl2‒2-MBIm metal-organic gel with variation in CuCl2 molar equivalent from 1.0 - 0.1 w.r.t. molar equivalent of 2-MBIm (From set I to set X).

(a)

2 µm

(b)

(c)

100 nm

Figure 3. FESEM images of the metallogel at (a) low and (b) high magnifications; (c) TEM image of the metallogel showing the gel fibrils.


Page 23 of 27

100000

100000

a

b G' and G'' (Pa)

G' and G'' (Pa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

10000

G' G'' 1000

100 1

10

10000

G' G'' 1000

100

100

10

Ang. frequency (rad/sec)

100

Oss.Stress (Pa)

Figure 4. (a) Storage (G′) and Loss (G″) modulus vs. angular frequency; (b) Storage (G′) and Loss (G″) modulus vs. oscillator stress at constant frequency of 1 Hz.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

Figure 5. Successive reduction of Cr(VI) to Cr(III) [(b) to (g)] at room temperature with a small piece of the freeze-dried metal-organic gel [(a)] when this is incubated in an aqueous solution of 10-2 M K2Cr2O7. The Cr(III) ions remains bound in the gel matrix which imparts the greenish colouration.


1000

Langmuir

2.0 (a)

0 min

Absorbance (a.u.)

30 min 60 min

1.5

90 min 120 min 150 min 210 min

1.0

270 min 330 min 390 min

0.5

0.0 300

400

500

600

Wavelength (nm)

(b)

5

2

R = 0.9947

-1

t/Qt (min mg g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

4 3 2 1 0 -1 0

200

400

600

800

1000

t (min)

Figure 6. (a) Time dependent kinetics of removal of Cr(VI) at room temperature with the freeze-dried metal-organic gel. (b) Pseudo-second-order linear plot for the removal of Cr(VI).


Page 25 of 27

(a)

5.5

1.6 2

R = 0.996

(b)

2

R = 0.432

5.0

1.2

lnQe

4.5

0.8

4.0 3.5

0.4

3.0 2.5

0.0 0

50

100

150

200

250

3.0

3.5

-1

4.0

4.5

5.0

lnCe

Ce (mg L )

Figure 7. (a) Ce/Qe versus Ce plot (Langmuir model). (b) lnQe versus lnCe plot (Freundlich model) for the removal of Cr(VI).

200

160 -1

Qe (mg g )

-1

Ce/Qe (g L )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

120

80

40

0

50

100

150

200

-1

Ce (mg L )

Figure 8. Adsorption isotherm of Cr(VI) at room temperature at pH~3.1.


250

5.5

Langmuir

350 300

-1

Qe (mg g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

250 200 150 100 50 2

3

4

5

6

7

8

pH

Figure 9. The effect of solution pH on the removal efficiency (adsorption capacity) of the gel sample.


Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Table of Contents Redox-Responsive Copper(I) Metallogel: A Metal-Organic Hybrid Sorbent for Reductive Removal of Chromium(VI) from Aqueous Solution Sougata Sarkar‡, Soumen Dutta‡, Partha Bairi§ and Tarasankar Pal*,‡ ‡

Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India

§

Polymer Science Unit, Indian Association for the Cultivation of Science, Kolkata-700032,

India E-mail: [email protected]


Redox-Responsive Copper(I) Metallogel: A MetalâOrganic Hybrid

Recommend Documents

Redox-Responsive Copper(I) Metallogel: A MetalâOrganic Hybrid